Paleotopographic Reconstructions in the Central Andes Based on Links Between Orographic Precipitation and Erosion

Abstract:

Links between topography and orographic precipitation have long been used to reconstruct paleotopography through the use of paleoenvironmental proxy data. In the eastern Central Andes, erosion rates are also coupled with precipitation rates, with spatial patterns in modern erosion rates closely matching precipitation patterns. Here we show how precipitation-linked variations in erosion can be used to reconstruct changes in regional topography and changes in the morphology of a range subjected to persistent orographic precipitation.

The Humahuaca Basin of NW Argentina lies leeward of an orographic barrier to easterly-derived precipitation. Stable isotopes and sedimentology from the basin fill indicate that once the humid basin was disconnected from the foreland during the Pliocene following growth of the Tilcara ranges to the east, a strong reduction in precipitation occurred. Cosmogenic in situ ¹⁰Be concentrations from the basin fill document a 10-fold decrease in paleo-erosion rates at ca. 3 Ma, coinciding with a strong decrease in precipitation reconstructed from stable isotopes. Moreover, paleo-erosion rates prior to 3 Ma closely match modern erosion rates along the humid eastern flanks of the Tilcara ranges. These results illustrate that close couplings among topography, precipitation, and erosion have persisted over millions of years, and thus may be useful for reconstructing other aspects of paleotopography.

One consequence of persistent orographic rainfall predicted by experimental data and supported by numerical models is progressive divide migration, but clear demonstrations of this process remain rare. In the Sierra de Aconquija, the easternmost range bordering the Andean foreland 400 km south of Humahuaca, precipitation on the western and eastern flanks differs by an order of magnitude. The distinct morphology of drainage basins on the western flank, with closely spaced, parallel outlets, has been argued to reflect progressive divide migration. While the morphological argument for divide migration is appealing, its rate and magnitude remain unconstrained. By combining thermochronology data with ¹⁰Be-derived, catchment-average erosion rates, we show that km-scale divide migration associated with persistent orographic precipitation has persisted over several million years.